1. Field of the Invention
The invention relates to pressure vessels or heat exchangers, and more particularly to a head closure apparatus for use with shell and tube-type heat exchangers.
In particular, the invention relates to an easily maintainable heat exchanger head closure apparatus suitable for cyclical modes of operation over rapidly changing thermal gradients, in that the apparatus is highly resistant to thermal stress cracking which has become a problem in heat exchangers presently being used in the electrical power generation industry, among others.
The invention also relates particularly to a compact and efficient retaining key apparatus for constraining the end-cover of an internally cylindrical heat exchanger head closure against internal working pressures.
2. Description of the Prior Art.
In prior shell and tube-type heat exchanger construction, two distinctly different types of head closure configurations have been known and used. The first type of known head closure construction is one having spherical internal and external geometry. Such spherical heat exchanger head closures possess the benefits of allowing relatively thin-wall construction under applicable ASME Code specifications, while retaining excellent strength characteristics so as to be highly resistant to thermal stress cracking caused by rapidly changing thermal gradients during start-up and shut-down of heat exchangers. However, spherical heat exchanger head closures are distinctly disadvantageous because access to the interior of the head closure generally is by means of a limited manhole opening, which does not allow for full access to the entire interior area of the heat exchanger tubesheet or to the entire interior volume of the head closure for necessary maintenance and repairs. As a result of this limited access opening, spherical heat exchanger head closure configurations generally are disfavored by many users of heat exchangers, including specifically the electric power generation industry.
The second type of heat exchanger head closure configuration previously known and used is one that has cylindrical internal and external geometry. Such cylindrical head closures generally have end-covers secured against internal working pressure by various means. An advantage of cylindrical head closures, as compared to spherical head closures, is that, when the end-cover of a cylindrical head closure is removed, full and complete access is provided to the entire interior volume of the head closure and to the entire interior area of the heat exchanger tubesheet. This full and complete access facilitates repairs and maintenance to the tubesheet and surrounding interior volume of the head closure. As a result, cylindrical heat exchanger head closures for many years have been widely favored and used by the electric power generation industry, among others.
One means known in the prior-art for securing the end-covers of cylindrical head closures involves a plurality of segmented annular retaining keys having multiple vertical-faced shear nodules that engage corresponding annular grooves inside the cylindrical head closure member. The disadvantage of such prior-art retaining keys is that the vertical-faced shear nodules must be spaced relatively far apart from each other, such that the retaining keys are lengthy and cumbersome and require that the overall length of the cylindrical head closure member must be increased to accommodate the shearing forces transmitted to the head closure member through the retaining keys. This in turn requires that the overall thickness of the end-cover must also be increased so that the access opening end-surface of the head closure member and the outer surface of the end-cover are maintained in a co-planar relationship in order to facilitate proper hydraulic sealing between the head closure member and the end-cover. As a result, the end-covers used with such prior-art retaining keys are substantially thicker and of substantially greater mass than otherwise required by applicable ASME Code specifications, which, together with the increased length of the head closure member, makes the cylindrical head closure apparatus unnecessarily expensive and difficult to manufacture and maintain.
It has developed that, prior to 1980, many electric power generating facilities economically were operated to meet base-load requirements, which resulted in such facilities operating for extended time periods without being removed from service. As a result, the large numbers of prior-art shell and tube-type heat exchangers having cylindrical head closures used by such electric power generating facilities generally were operated in substantially continuous modes involving relatively lengthy periods of steady-state operation, with such heat exchangers being only randomly and infrequently taken out of service. As a result, the heat exchangers with cylindrical head closures installed in many older electric power generating facilities generally were designed and suitable for relatively constant service, as opposed to daily start-up and shut-down cyclical modes of operation.
Today, however, many of these older electric power generating facilities that originally were designed for base-load continuous service now are being used as "peaking" facilities, which requires that such facilities be started-up and partially or totally shut-down on daily or other relatively short cyclical frequencies. As a result of such cyclical operation, it has developed that the prior-art cylindrical heat exchanger head closures used in those older electric power generation facilities are incapable of withstanding such cyclical operation, in that the rapidly changing thermal gradients caused in such cylindrical heat exchanger head closures by such cyclical operation have been shown to result in substantial thermal stress cracking of such cylindrical head closures. Such thermal stress cracking most commonly occurs at the points of structural discontinuities between the barrel walls of the cylindrical closure members and the tubesheet surfaces of the head closures. These thermal stress cracking problems have resulted in extended outages of numerous electric power generating facilities, with attendant large losses in revenue and greatly increased maintenance expenses to the electric power generation industry.
Although prior-art spherical heat exchanger head closures generally are capable of withstanding the severe thermal stresses caused by cyclical operation of electric power generating facilities, the owners of such facilities largely have chosen not to install spherical head closures due to the limitations on access to the interiors of spherical head closures and the attendant difficulty of maintenance and repairs, as described above.